1. Field of the Invention
This invention relates to filtration media, and more specifically to a high capacity adsorbent and the fabrication thereof. The high capacity adsorbent includes enhanced performance towards fluoride ion and oxyanions of arsenic and phosphorus in a broad pH range in the presence of high excess of competitive ions. A high capacity adsorbent is useful for the selective removal of fluoride ion and oxyanions of phosphorus and arsenic from drinking water, industrial streams and wastes, in the medicine and food industries.
2. Description of Related Art
Ion exchange properties of insoluble polyvalent metal hydroxides have been studied since the 1950's [C. B. Amphlett, Inorganic Ion Exchangers, Elsevier, New York (1964)]. It was found that some of these hydroxides (aluminum hydroxide, ferric hydroxide, titanium hydroxide, zirconium hydroxide, and the like) are amphoteric and behave as cation exchangers or anion exchangers depending on pH.
Fluoride is considered an essential element for animals and humans because of the role it plays in bone and dentin mineralization. However, when fluoride is present in excess of 1.5 mg/L it affects teeth, endocrine glands, liver and other organs. Presently, the World Health Organization recognizes activated alumina (AA) adsorption as one of the best adsorbents for water defluoridation. A number of studies on AA performance in water defluoridation under varying process conditions have shown that the optimum pH for maximum adsorption is between 5 and 7. The capacity of the activated alumina on fluoride depends on the AA preparation route, crystalline form, and can vary from 1 to 20 mg/g, depending on the operating conditions [A. Bhatnagar et al, Fluoride removal from water by adsorption—A review, Chemical Engineering Journal, 171, p. 811-840, (2011)]. Other types of polyvalent metal oxides also have ability to remove fluoride ion, but their efficiency typically is lower than that of AA. Granular ferric hydroxide (GFH) demonstrates capacity on fluoride of approximately 7 mg/g at a pH of approximately 6-7; hydrous titania and zirconia have similar poor performance (U.S. Pat. No. 6,077,809, demonstrating capacity of approximately 5 mg F/g at a pH of approximately 3). Some binary polyvalent mixed oxides have been tested as potential adsorbents for fluoride. It was found that alumina-titania oxide has a capacity of approximately 3 mg F/g; zirconia-iron oxide having a capacity of approximately 10 mg F/g; and alumina-iron oxide having a capacity up to 4 mg F/g. (See, for example, U.S. Pat. No. 6,599,429). Alumina-ceria oxide has high capacity on fluoride up to 90 mg/g at a pH of 6, but the high cost of cerium makes this material cost-prohibitive for water defluoridation.
Polyvalent metal based hydrous oxides show high affinity towards oxyanions of phosphorus and arsenic. Zirconium hydrous oxide has been used for phosphate removal in kidney dialysis. (See, e.g., U.S. Pat. Nos. 6,878,283; 7,033,498; 7,135,156; 8,002,726; and U.S. Patent Publication No. 2010/0078387). It has been shown that zirconium oxide capacity on phosphorus from blood dialysate can reach 90-120 mg PO4/g. High affinity of iron oxides to PO4 allows their use for water treatment (Ref. U.S. Pat. No. 5,876,606), as well as for oral administration in humans and animals for hyperphosphatemia prevention (Ref. U.S. Patent Publication Nos. 2009/169645 and 2013/316018).
Aluminum hydroxide was proposed as an adsorbent for arsenic (Ref. U.S. Pat. No. 6,030,537); however, alumina capacity on arsenic is significantly lower than that of hydrous titanium (see, e.g., U.S. Pat. Nos. 6,919,029; 7,473,369; and 7,497,952), zirconium (U.S. Pat. Nos. 6,383,395; 7,252,767; and 6,077,809) and iron oxides (U.S. Pat. Nos. 6,830,695; 7,625,487; and 8,404,210). Moreover, aluminum hydroxides are able to adsorb only arsenic(V) and have no affinity towards arsenic(III). Using polyvalent mixed oxides for selective arsenic removal has been shown demonstrated: alumina-iron oxide (U.S. Pat. No. 6,599,429), manganese and iron-doped titania- and zirconium-oxides (U.S. Pat. No. 8,216,543).
Broad use of selective inorganic adsorbents for water defluoridation and arsenic removal, eutrofication prevention, treatment of industrial streams, and in different applications in the medicine and food industries is absent in the art, and requires further investigation for cost-efficient materials with enhanced adsorption performance.